The present invention relates generally to sterilization of instruments and, more particularly, to the combined application of microwave and thermal energy in the presence of steam and liquid water to the sterilization of dental and surgical instruments and other objects.
Several instrument sterilization procedures are presently in use. Autoclaving is most commonly employed, but slowly dulls sharp metal instruments. In U.S. Pat. No. 4,865,814 for xe2x80x9cAutomatic Sterilizerxe2x80x9d which was issued to Bobby B. Childress on Sep. 12, 1989 a microprocessor-controlled heater which generates steam inside of a sealed chamber is described. The pressure level rather than the temperature is used to control the heater. Air is caused to be displaced from the sterilization chamber by the generation of the steam; however, this process does not remove all of the air. The presence of air interferes with production and maintenance of steam at the optimally desired temperature and pressure in the chamber and causes corrosion. Instances of failed sterilization using steam sterilizers are common. Such instances may be triggered by admixture of steam by trace air.
In xe2x80x9cA Report Of An Outbreak Of Postoperative Endophthalmitisxe2x80x9d by W. Swaddiwudhipong et al., J. Med. Assoc. Thailand 83, 902 (2000) defects in surgical sterilization including possible inadequacy in the autoclave sterilization of surgical instruments is reported. In xe2x80x9cThe Use Of Autoclaves In The Dental Surgeryxe2x80x9d by N. W. Savage and L. J. Walsh, Australian Dental Journal 40, 197 (1995), the authors state that although autoclaving is the absolute method of achieving instrument sterilization in any health-care setting, its effectiveness relies on an effective pre-sterilization routine for instrument handling and the subsequent correct loading and operating of the autoclaves. Similar findings are reported in xe2x80x9cAutoclave Performance And Practitioner Knowledge Of Autoclave Use: A Survey Of Selected UK Practicesxe2x80x9d by F. J. T. Burke et al., Quintessence International 29, 231 (1998). In xe2x80x9cDisinfection And Sterilization Practices In Mexicoxe2x80x9d by M. Zaidi et al., J. Hospital Infection 31, 25 (1995), the authors report the use of too short an exposure time in steam sterilizers or dry heat sterilizers as contributing to ineffective sterilization of surgical instruments.
Heat sterilization at approximately 160xc2x0 C. is also used. However, this method requires heat generators capable of rapid heating which are not commonly available, and rubber and plastic parts may be damaged. Chemical sterilization techniques have the disadvantage that hazardous materials such as ethylene oxide or alkaline glutaraldehyde must be handled and disposed of in a hospital or dental clinic environment. Moreover, sterilization times are lengthy.
Sterilization of medical and dental instruments by directly and indirectly using microwaves is known. In both U.S. Pat. No. 5,019,359 for xe2x80x9cMethod And Apparatus For Rapid Sterilization Of Materialxe2x80x9d which was issued to Barry S. Kutner et al. on May 28, 1991 and U.S. Pat. No. 5,039,495 for xe2x80x9cApparatus For Sterilizing Articles Such As Dental Handpiecesxe2x80x9d which was issued to Barry S. Kutner et al. on Aug. 13, 1991, a liquid sterilant solution and the material to be sterilized are placed in a sealable, vapor-impermeable collapsible pouch. Microwave energy vaporizes the sterilant solution and the instruments are exposed either to the vaporized sterilant alone or to both microwave radiation and the vaporized sterilant. The vaporized sterilant prevents arcing and assists in sterilizing the instruments when used in conjunction with the microwaves. In U.S. Pat. No. 5,417,941 for xe2x80x9cMicrowave Powered Steam Pressure Generatorxe2x80x9d which issued to Bernard A. McNulty on May 23, 1995, an apparatus which produces high temperature and pressure steam derived from microwave energy is described. Microwave energy is coupled into a guiding structure such that essentially all of the energy is transferred to a reaction fluid contained in a holder located at the end of the guiding structure. The reaction fluid is rapidly vaporized and the resulting vapors expand into a high-pressure chamber through a metal screen that also prevents transmission of microwave energy. No mention is made of whether the resulting temperature and pressure permit steam sterilization to occur, whether the sterilization chamber is free of air during the sterilization cycle, or whether arcing of the metal parts is avoided.
In xe2x80x9cNonthermal Killing Effect Of Microwave Irradiationxe2x80x9d by Seigo Sato et al., Biotech. Techniques 10, 145 (1996), the elucidation of the lethal effects of microwave radiation at constant temperatures is described. It was found that the death rates for E. coli exposed to microwave irradiation were higher than those obtained in conventional heat sterilization at the same temperatures. In xe2x80x9cHeat Transfer Analysis Of Staphylococcus aureus On Stainless Steel With Microwave Radiationxe2x80x9d by C. B. A. Yeo et al., J. Appl. Microbiol. 87, 396 (1999), the authors show that the microwave killing pattern of Staph. aureus is principally due to heat transfer from the stainless steel substrate which absorbs microwave energy in the surface regions, and that little direct energy is absorbed by the microbes from the incident microwave radiation. Complete bacterial inactivation was achieved at 61.4xc2x0 C. with an irradiation time of 110 s.
Metallic instruments are problematic in microwave-assisted sterilization processes because such instruments reflect microwave energy and, when placed in microwave field, will arc. In U.S. Pat. No. 5,599,499 for xe2x80x9cMethod Of Microwave Sterilizing A Metallic Surgical Instrument While Preventing Arcingxe2x80x9d which was issued to Jeffery S. Held and Robert F. Schiffmann on Feb. 4, 1997, and in U.S. Pat. No. 5,607,612 for xe2x80x9cContainer For Microwave Treatment Of Surgical Instrument With Arcing Preventionxe2x80x9d which was issued to Jeffery S. Held and Robert F. Schiffmann on Mar. 4, 1997, a container for preventing arcing of a metal object placed therein and subjected to microwave radiation is described. To reduce arcing between metal surgical instruments, the container includes a tray upon which the instruments are located a suitable distance apart. Moreover, the container has at least one surface for absorbing microwave energy which impinges on the exterior surfaces of the container for converting the absorbed microwave radiation into heat that sterilizes the instruments. Iron oxide (that is, Fe2O3) materials are employed for this purpose, and prevent substantially all of the microwave radiation impinging on the exterior surface of the container from entering the volume of space therein.
In U.S. Pat. No. 4,861,956 for xe2x80x9cMicrowave/Steam Sterilizerxe2x80x9d which issued to Calice G. Courneya on Aug. 29, 1989 a microwave/steam sterilizer is disclosed. The authors state that the sterilizer hydrates potential pathogens, including spores, and subjects them to relatively uniform electromagnetic energy without arcing and without self-destruction of the microwave source from reflected microwave energy. According to Courneya et al. microwave energy is used to vaporize water forming steam which is rapidly absorbed by dry spores making them vulnerable to killing by direct microwave energy. The water vapor also keeps electrical charges sufficiently low that arcing and sparking are overcome. The sterilizer provides an adequate availability of water, as steam, to allow the dry spores to hydrate without flooding with excess water which acts as a coolant and prevents the formation of super-heated steam internally within the spores. Expected sterilizer temperatures are in the region of 98.9xc2x0 C. Excess steam is preferentially attracted to the coolest area in the chamber, namely, the chamber walls. Thus, liquid water is not present on the instruments being sterilized. Additionally, 98.9xc2x0 C. and steam at near atmospheric pressure are inadequate for sterilization of instruments based on experience with autoclaves.
In xe2x80x9cA Microwave Based Device For Sterilisation/Disinfection Of Surgical And Dental Equipmentxe2x80x9d by Peter Nielsen et al., PCT/DK00/00146 having an International Publication Date of Oct. 5 2000, the inventors describe two embodiments of a microwave-based sterilizer. In one embodiment, tools are placed on trays and irradiated using microwave radiation. In the second embodiment, tools are placed in a sealed chamber which is placed in a volume into which microwaves are introduced. A steam condenser in fluid communication with the chamber is located outside of the volume and permits steam generated from a water reservoir within the chamber to be removed from the chamber, condensed and returned to the reservoir to enable the generation of additional steam. A desired pressure of 3 atm at 130xc2x0 C. is taught.
In xe2x80x9cInfluence Of Moisture On Microwave Arcingxe2x80x9d by Elias J. Abou-Kasam et al., International Symposium on Microwave Technology In Industrial Development, Brazil, Jul. 22-25 1985, ANAIS Proceedings, pages 393-396, 13100-Campinas, Sao Paulo, Brazil, the authors state that moist air reduces the electric field around metallic objects; the higher the humidity, the higher the threshold microwave power for arcing. The effect on the electric field of a liquid water layer on the metal objects is not discussed.
As will be discussed hereinbelow, if a film of water (dielectric constant, xcex5water, ≈46 at 135xc2x0 C. and 47 psi at 2.45 GHz) is present on the metallic object, for the same incident microwave power the electric field experienced by the object will be significantly reduced when compared with the electric field expected for the same object disposed in an environment of superheated steam (xcex5steam ≈1.01) at the same temperature, pressure and microwave frequency (see, e.g., F. Buckley and A. A. Maryott in NBS Circular 589 (1958)). From page 41 of Dielectrics and Waves by A. von Hippel, Wiley, N.Y. (1954), the microwave power equation is given by P xe2x88x9d xcex5E2, where xcex5 is the dielectric constant of the medium in which microwave power, P, is incident and E is the electric field therein. From this equation one may observe that for the same applied microwave power, E2steam/E2water=xcex5water/xcex5steam≈46/1.01, where Esteam is electric field experienced by a metal object in the presence of steam and Ewater is the electric field experienced by that same object when coated with water, from which Esteam/Ewater≈7. Thus, the electric field experienced by a water coated object is seven times smaller than that experienced by the same object in the presence of steam, and the likelihood of arcing is correspondingly reduced.
Accordingly it is an object of the present invention to provide an apparatus and method for effectively sterilizing surgical and dental instruments using a combination of microwave radiation and thermal energy in the presence of both steam and liquid water without arcing.
It is also an object of the present invention to provide an apparatus and method for effectively sterilizing surgical and dental instruments using a combination of microwave radiation and thermal energy in the presence of both steam and liquid water without arcing, where air is substantially removed from the vicinity of the instruments before the sterilization process.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the sterilization apparatus hereof includes: a sealed chamber capable of withstanding internal pressure and vacuum and having a sealable opening for introducing and removing items to be sterilized; a tray disposed within the chamber for holding items to be sterilized; at least one microwave radiation generator; microwave waveguide for directing microwave radiation generated by the at least one microwave radiation generator onto the items to be sterilized in said chamber; a pump for removing air from the chamber before microwave energy is directed onto the items to be sterilized; means for generating steam at greater than one atmosphere of pressure in the chamber; and a sprayer for directing droplets of water onto the items to be sterilized.
Preferably, means are provided for removing steam from the chamber after the sterilization process.
Preferably also, means are provided for cooling the items to be sterilized after the sterilization process.
In another aspect of the present invention in accordance with its objects and purposes the sterilization method hereof includes the steps of: placing the items to be sterilized in a sealed chamber capable of withstanding internal pressure and vacuum; evacuating the chamber to remove substantially all of the air therein; generating steam at greater than one atmosphere of pressure in the chamber; directing pulsed microwave radiation onto the items to be sterilized; and directing droplets of water onto the items to be sterilized.
Preferably, the steam is removed from the chamber after the sterilization process is completed and the items to be sterilized are cooled.
Preferably also, removing the steam from the chamber after the sterilization process is completed and cooling the items to be sterilized is accomplished by flowing dry nitrogen over the items to be sterilized.
Benefits and advantages of the present invention include rapid and complete sterilization of surgical and dental tools and other appliances without arcing.